The present invention relates to a navigation system on a spacecraft and, in particular, to an optical navigation sensor for use in such a navigation system.
The optical navigation sensor is used to take pictures of celestial objects such as the moon or a particular planet and particular two or more stars so as to obtain the information of the direction of the particular planet or the moon viewed from the spacecraft in an inertial space. The information obtained is used to improve the spacecraft's orbit determination accuracy.
In the paper entitled "EXPERIMENTAL OPTICAL NAVIGATION AND GUIDANCE FOR MUSES-A", 39th congress of the INTERNATIONAL ASTRONAUTICAL FEDERATION, Oct. 8-15, 1988, Bangalore, India, T. Nishimura et al disclose an optical navigation sensor which comprises a hood for preventing the sunlight from getting into an optics of the sensor, the optics for taking images of the moon and stars, and an electronics to convert the image data into serial digital signals.
The optics includes a Gaussian type lens system, two two-dimensional charge-coupled devices (CCDs) and a part of the electronics.
The two CCDs of the same type are assembled on a focal plane, one for the moon and the other for the star detection. As the moon is much brighter than stars, a light attenuation filter and a slit are implemented to reduce the moon's light to an adequate intensity level for the CCD.
The moon and star lights collected by the lens system are focused on the CCDs. Photo electrons are induced in each CCD pixel by the moon and star lights and are time-delayed and integrated (TDI) synchronously with the spacecraft spinning to prevent the images from smearing due to the images translation motion on the CCDs. The TDI clock (charge shift clock) can be tuned accurately to the spacecraft spin-rate from the ground.
One of the two CCDs' output signals is selected by an external command to the electronics. After a dark-current subtraction, image signals are A/D converted into digital signals and then stored into a memory. The data stored in the memory is reproduced and transmitted to the ground station.
To achieve the determination accuracy of the moon's direction viewed from the spacecraft in an inertial frame, the sensor also detects stars and determines the moon's direction by using the stars detected by the sensor itself as celestial references, and thus avoid the degradation in the accuracy due to the spacecraft wobble and the sensor misalignment with respect to the attitude sensors such as a spin type sun sensor, an earth sensor, and a star scanner.
In the known optical navigation sensor, since two CCDs are used for the moon and the stars, two CCD drivers must be used and a selection switch must be provided for selecting one of the two CCDs' outputs. This results in the electronics which is complex in structure, large in size and power consumption, and expensive in cost.
Further, two dimensional CCDs are positioned not on the optical axis but at both sides of the optical axis. Accordingly, the lens system is required to have a large aperture sufficient to make images on the two CCDs positioned adjacent to each other at the focal plane. Further, the lens system is required so that the images are made on the CCDs without distortion. This makes the lens system complex and large in structure, and expensive in cost.